Posttranscriptional modifications in RNA play a variety of important roles, including stabilization of tertiary structure and enhancement of fidelity of intermolecular interactions in protein synthesis. Determination of the structures and sequence locations of modified nucleotides are essential steps in understanding modification - function relationships. Taking advantage of recent advances in mass spectrometry methodology, it is proposed to study posttranscriptional modification patterns in bacterial and archaeal ribosomal RNAs, and in transfer RNA. These studies will contribute to a clearer understanding of the structure and function of the ribosome during protein synthesis, and of structural motifs that influence RNA stability, an important element in design of antisense- and ribozyme-based therapeutic agents.